Shelf Sea Biogeochemistry blog

Wednesday, 15 April 2015

Spring has sprung - here comes the bloom

Alex Poulton,
National Oceanography Centre

After two
weeks in the Celtic Sea we are seeing clear signs that the spring bloom has
truly begun - nutrients are declining whilst levels of the pigment chlorophyll,
used by phytoplankton for photosynthesis, are steadily rising.

Just how green the water is at present (slightly cheating as this is a
pigment extract rather than seawater). Photo: Chata Seguro.

The
bloom appears to be patchy across the Celtic Sea; from the shelf edge where the
bloom has not started to show strongly yet, to the central Celtic Sea (where
our Candyfloss site is) where small phytoplankton are actively growing, to the
northern Celtic Sea where we saw huge diatoms (images below) - a type of
phytoplankton which often characterises blooms and productive waters - which
were at least a hundred times larger than anything we have seen so far.

Diatoms and
zooplankton seen under the microscope. Photo: Chata Seguro.

A close up of one of the large diatoms we saw in the NE Celtic Sea.
Photo: Chata Seguro.

As the
nutrient levels continue to decline we are keen to see what happens within the
phytoplankton community: will there be a clear progression from large cells to
smaller cells which needs less nutrients for growth, will the diatoms be
succeeded by another phytoplankton group? How these changes are reflected in
the rest of the ecosystem is a key question we will address over the next two
weeks. For example, how will changes in which type of phytoplankton is present
influence the different nutrients needed for their growth (nitrogen,
phosphorus, silica), and will we see changes in the dominant types of
zooplankton (tiny animals that eat the phytoplankton) across the Celtic Sea.

The ever present fog viewed from the bow of the RRS Discovery. Photo:
Chata Seguro.

Though the
bloom has arrived, we have lost the sun - a dense sea fog has
descended on us over the last few days which means we can only see a hundred to
two hundred metres in any direction (see image). The eerie silence that this
has brought to the ship is broken up at regular intervals by the ear shattering
sound of the ships horn announcing our presence. If the spring bloom didn’t
know we were here before, you can be sure that it does now.